1. Field of the Invention
The present invention relates to a liquid crystal compound useful as a material for an optical device, and a liquid crystal medium, and in particular, to a liquid crystal compound having a large dielectric anisotropy, a large refractive index anisotropy, and a low melting point; and a liquid crystal medium having a liquid crystal phase over a wide temperature range, a large dielectric anisotropy, and a large refractive index anisotropy. In addition, the present invention further relates to an optical device using the liquid crystal medium, and in particular, to an optical device that can be used over a wide temperature range and driven at a low voltage, and can achieve a high-speed electro-optical response.
2. Description of Related Art
Liquid crystal display (LCD) devices containing liquid crystal compositions are widely used as the display for clocks calculators, word processors, and so on. These LCD devices utilize the refractive index anisotropy and dielectric anisotropy of liquid crystal compounds. It is known that the operation modes of the LCD devices mainly include phase change (PC), twisted nematic (TN), super twisted nematic (STN), bistable twisted nematic (BTN), electrically controlled birefringence (ECB), optically compensated bend (OCB), in-plane switching (IPS), and vertical alignment (VA) which utilize one or more polarizers for display. Moreover, in recent years, more attentions have been paid to the mode in which an electric field is applied to an optically isotropic liquid crystal phase to make the liquid crystal phase to exhibit electrically controlled birefringence (Patent Documents 1-9, and Non-patent Documents 1-3).
Moreover, wavelength variable filters, wavefront control devices, liquid-crystal lenses, aberration correction devices, aperture control devices, and optical head devices that utilize the electrically controlled birefringence in a blue phase, as one of the optically isotropic liquid crystal phases have been proposed (Patent Documents 10-12).
According to the driving mode, the LCD devices can be classified into passive matrix (PM) type and active matrix (AM) type. The PM type is further classified into static type and multiplex type, and the AM type is further classified into thin film transistor (TFT) type, metal insulator metal (MIM) type and so on.
Such LCD devices contain a liquid crystal composition having suitable physical properties. To improve the characteristics of an LCD device, the liquid crystal composition preferably has suitable physical properties. A liquid crystal compound as a component of a liquid crystal composition requires to have the following general physical properties:
(1) stable chemical and physical properties;
(2) a high clear point (i.e., phase transition temperature from the liquid crystal phase to the isotropic phase);
(3) a low lower-limit temperature of a liquid crystal phase (i.e., an optically isotropic liquid crystal phase such as nematic phase, cholesteric phase, smectic phase, and blue phase);
(4) excellent compatibility with other liquid crystal compounds;
(5) a suitable dielectric anisotropy; and
(6) a suitable refractive index anisotropy.
Particularly, for an optically isotropic liquid crystal phase, a liquid crystal compound having a large dielectric anisotropy and a large refractive index anisotropy is preferred, from the viewpoint of lowering the driving voltage.
When a liquid crystal composition containing a liquid crystal compound having stable chemical and physical properties as described in (1) is used in an LCD device, the voltage holding ratio is improved.
Further, a liquid crystal composition containing a liquid crystal compound having a high clear point or a low lower-limit temperature of a liquid crystal phase as described in (2) and (3), can have a nematic phase or an optically isotropic liquid crystal phase over an expanded temperature range, thus being capable of being used in a display device over a wide temperature range. A liquid crystal composition is generally prepared by mixing a liquid crystal compound with a number of other liquid crystal compounds, so as to exhibit better properties that are difficult to develop by a single compound. Therefore, a liquid crystal compound having good compatibility with other liquid crystal compounds as described in (4) is preferably used in an LCD device. In recent years, LCD devices with superior properties, especially display performances, for example, contrast, display capacity, and response time, are especially required in the industry. In addition, regarding the liquid crystal material that is being used, a liquid crystal composition having a low driving voltage is required. Furthermore, in order to drive an optical device driven in an optically isotropic liquid crystal phase, a liquid crystal compound with a large dielectric anisotropy and a large refractive index anisotropy is preferred.
It was reported that chlorobenzene derivatives analogous to the compound of the present invention have large dielectric anisotropy and large refractive index anisotropy (Patent Document 13), but a compound having a chlorobenzene moiety and the excellent properties of the compound according to the present invention have not been reported. Moreover, as for the optically isotropic polymer/liquid crystal composite material disclosed in Patent Documents 1-3 and Non-patent Documents 1-3, the device needs to operate at a high voltage. Although an optically isotropic liquid crystal composition and a polymer/liquid crystal composite material, which are expected to have an operation voltage lower than the voltages as disclosed in Patent Documents 1-3 and Non-patent Documents 1-3, are disclosed in Patent Documents 4-9, an optically isotropic liquid crystal composition containing a compound having a chlorobenzene moiety of the present invention and polymer/liquid crystal composite material were not disclosed.